Abrasive product and method of making the same



Patented Nov. 3, 1931 um'rso s'rArss PATIENT orr ca LEWIS E. SAUNDERS AND K. mmeamor WOW AID m'rorr I. BEEOHER, OI HOLDEN, IASGACHUSET'IS, ASBIGNOB! IO HORTON OOIPAIY, OI WOEOES'IER, HABGAC EUSBTI'B, A CORPORATION OI PRODUCT AND m0!) OI m6 m II No Drawing.

This invention relates to abrasive products and more particularlyto a grinding wheel made of abrasive grains bonded by a ceranuc material.

A grinding wheel of this ty is ordinar ly made of-abrasive grains suc as crystalline alumina or silico'n carbide, united by a bond of ceramic material fired at a high temperature to form a vitreous or vitrified mass. Such a bond is ordinaril composed of a mixture of one or more c ays, such as ball clay and sli clay, with feldspar, kaolin, flint and the like,'and when matured is similar to porcelain or glass. There are a large number of bond compositions utilized in this field, depending upon the required ade of wheel hardness and the nature an size of the abrasive grains used, as well as the other desired characteristics of the wheel. The bond selection has heretofore de nded to a large extent upon unscientific it or miss methods, and in spite of many years of ractical' experimentation in tryin out such nd compositions, the grinding w eels often display unexpected and undesired characteristics. and two wheels made of the same composition and supposedly by the same method may not be at all alike 1n their griding propertles. The grade of hardness of a wheel,

or its apparent grade under grinding con-= ditions, is measured largely by the strength with which the bond holds the abraslve grains in position under the stresses of grindmg or of testing the wheel. A softer grade of wheel, having a low strength of bondin ordinarily wears away faster during grin ing than does a hard wheel. Yet, in spite of painstaking efforts by the wheel makers to produce uniform products, it is sometimes found that a wheel of supposedly harder grade Wears away faster than one predetermined to be a softer wheel. Then again, some grinding wheels drop in their modulus of elasticity as they age, probably because of the gradual yielding to certain internal strains. Stability of the properties of the wheel is an important and desired characteristic.

A study of such problems as these shows that various factors heretofore neglected Application fled August 8, 1987. lerlal Io. $11,011.

must be considered. With crystalline aluminum oxide abrasives, for instance, the contamination of a bond of supposedly uniform composition byimpeurities and alumina from the abrasive must considered. This is due to the fact that the vitrification recess ordinarily requires a temperature 0 1200 G. or higher, and it is found that at such a of temperature the slag, and glassy im unties present in the abrasive may exu e at the surface of the grain and mingle with the bond composition. In addition to slag exudation the composition of the bond may be somew at altered to a deleterious extent by actual chemical attack of the molten bond on the surface of the abrasive grain. There may be as high as 82 times as much abrasive as there is bond in a given wheel mixture hence the presence of but a small amount of slag in the abrasive ma materially affect the bond composition, an the efiect'will vary depending upon the amounts of the. ingrealumina eontributed'b bond will also do n on the temperature andconditions of g, as well as the particular abrasive and bond used. It is ob- 1100 C. and it is therefore important to fire the wheels at temperatures ap recia' bly below 1200 G. in or er to eontrolorto eliminate this condition. It'ismry, however, that with any given'bond, a suflicient firing temperature be used to give the bond enough fluidity so that it will come thoroughly into contact with the grain surfaces. It is observed also, that decomposition of silicon carbide presents similar dificulties in that the ingredients thereof enter and affect the bond composition.

Another difliculty lies in the fact that many of the vitrifiable bonds ordinarily employed in the manufacture of grinding wheels go through a swelling stage during the process of vitrification, and it is necessary that the wheel be porous enough so that the bond may swell into the pore spaces between the abrasive grains. This makes difiicult the manufacture of wheels containing relatively large dients employed. The amount of slag and the abrasive to the .aan'mf bond. In the case of-a'luminous: abrasive, this condition is probably .due to the fact that the bonds inaril contain an iron. compound, which is usua y present as ferric oxid. This, and'other ingredients I acting to a lesser degree, .d1ssoc1ateor go through other chemical changes-atthe maturing temperature to which such bonds are sub'ected, with a resultant'evolution of as his an a consequent swelling the bond. ta

e ma ur m- -may take place rapidly i a pgrature is higher than 12002-O., an such require long-lived fast. cutting I may be rotated at'vei'y high speeds, etthe- 'nds must be fired under such conditions thatthey go entirely through the -swelling stage and ultimately subside as a glass; but

- the swelling may cause disarrangement of the} abrasive grains, and otherwise change the wheel'characteristics to a detrimental extent. -Although modern grinding-loperations w eels which problems above noted have made it 'fiicult to-manufacture such wheels, and particularly to pass through ascreen of 16 meshes to the those made of grit sizes which are too large linear inch.

-In thecourse ofour investigation of such wheels we have observed a strained condition at the point of'union between the glass bond and the grains of abrasive, and in many cases the development of fine cracks in the glass bond is readily apparent. These cracks may form either during the manufacturing operation, or as the wheel a es during use or disuse. The'coefiicients 0 expansion of aluminous' abrasives and of various bonds have been studied, and we have found that the bonds'ordinarily used do nothavethe same coeflicients of expansion as the abrasive grains, and that consequently the customary operations of making a vitrified wheel result in setting. up states of strain between such bodies:'" 'A difference in fit between the bond and the abrasive may strain the bond to the point where cracks develop in it, and in this condition the bond is said to be crazed. The strength of a crazed bond is inuch less than that of thesame bond in the uncrazed condition. This causes an excessive wearing changes in the-composition of the glass due to the separation from. it of crystals of a different composition from the ori inal glass. We have applied the term fcrackling tothis use for making grinding 'crtckea s ndman 0f the beta when the eye tals ar involved. .QIt is obvious that crystals ,ofdifi'e ent kinds may act entirely difierently in this-respect, dependent-on their chemical composition, size, expansivitytand other fac-;

tors." I H H .Asan example'of a'raw bond in common wheels in which the abrasive isorystallinea umina, the following This composition matures glass at a temperature in the vicinity of 1200 to 1300 C. An appreciable amount of iron oxid is present and this causes the bond to swell when fired, forming oxygen gas as the result of thermal dissociation above'about 1175 to 1225 C. Such a bond first'shrinks and sinters to a dense body, then it begins to soften and when sufficiently fluid it swells as gas is evolved, and finally it. subsides to a dense into a grindblack glass. When made'up b umina a rasive ing wheel with. crystalline a of the regular variety,which may contain about 95% of alumina and various impurities, such as ferric oxid, sllica and titania, a

I wheel is produced which has many undesired characteristics. Such a wheel is to a certain extent unstable, in that its ph sical properties may change as it ages. T e wheel may be soft in grade of hardness and weak compared with the strength that it should have.

There is often found in such a glass bond a considerable development of plagioclase feldspar crystals and mullite crystals. Also, the.

bond in such a wheel is frequently found in a strained condition which results in crazin or y crackling. This bond glass has a coefiicient of expansion of about 45x10", whereas crystalline alumina grains have an average coefiicient of expansion of about 72x10. A bond glass of this type may contain some undissolved material of the original raw mixture as well as crystals which have been formed from the materials of the bond, either alone or together with those substances contributed by the abrasive. We believe that the feldspar crystals are particularly detrimental in their tendency to cause crackling, but that a small quantity of the fine mullite needles sometimes observed in such a bond are not deleterious. The strains in such aglass bond may be released and cause crazing and crackling at any time'from the period of firing the wheel to years later, and this formation of cracks may be accelerated by the resence of moisture,.or perhaps may be starte by shocks or scratches or thermal changes as well as by other conditions which are incident to the stora e and aging and use of the gr nding wheef. These disadvantages are particularly noticeable in wheels havingla grit size of abrasive coarser than that w ich wilhpass through a screen of 60 meshes to the lmear inch. The larger grain sizes cause the greater trouble, due primarily to the larger area of contact between the grain of abrasive and the bond. I I I The chief objects of our invention are accordingly to overcome such difiiculties and in particular to rovide a method of making an abrasive pro not containing bonded granular abrasive material which will not swell detrimentally during the manufacturing opoperation 'or craze or crackle as it ages or is used. Further objects are to provide such bonding ingredients and to so carry on the manufacturlng procedure as to obtain products of ceramic bonded granular material which avoid the undesirable characteristics specified, and in particular to provide grinding wheels or other ceramic bodies of this type which are stable, and not swelled, crazed or crackled toa detrimental extent and which may be made in the extremely hard and soft grades throughout a wide range of grain sizes and degrees of porosity.

With these and other objects in view, as will be apparent to one skilled in the art, this invention resides in the composition of matter as well as the process of making the article herein described as covered by the claims appended hereto.

In accordance with our discoveries and as the result of our experimentation, we have found that a grinding wheel should be made up of such materials that the abrasive grains and the bond will have substantially the same coefiicients of expansion throughout a long range of temperature change and especially below the temperatures of softening or annealing of the matured bond, it being noted that at high temperatures the bond is capable of permanently deforming and yielding to the strains set up as the body cools. If the abrasive grains and bond expand and contract alike, then the formation of. internal strains at the surface of the grains will be thereby minimized. It is important that crackling as well as crazing be minimized and this is accomplished by utilizing such bond compositions and so carrying out the vitrification process as to minimize, in so far as possible, the formation Within thebond of an apprecible amount of crystals of sizes and types which have been found to be deleterious. It is desirable that the bond be cooled rapidly through the range'in which crystals ma form, and if desired the bond may be coo ed slowly .below this range during the annealin period. We also prefer to employ a bond or a inding wheel, which is substantially insolub e under ordinary conditions of usage in the liquids employed to cool or lubricate the wheel and work. We have found that the bond should be so constituted that it will not swell detrimentally during the firing operation and that the bond should mature at a temperature sulficiently low so. that the abrasive ingredients will not contaminate the bond to any considerable extent. The temperature at which the bond should mature, if the abrasive is crystalline alumina, should be below 1200 (1, and preferably below Seger cone 6, it being noted that the slag and other impurities in the alumina abrasive tend to come out but little at temperatures below 1100 C.

bond composition for each abrasive which has these desirable characteristics may be readily picked out by one skilled in the art in view of the above disclosure. As an illustration of one bond which is serviceable for use with an abrasive consistin largely of crystalline alumina, we may emp 0y a bond mixture of the following percentage composition:

Silicon dioxid 55. 0 Aluminum oxid 11. 4 Ferric oxid 4. 3 Magnesium oxid 2. 8 Calcium oxid 5. 9 Sodium oxid 6. 0 Potassium oxid 2. 6 Titanium oxid 0. 7 Boron oxid 11. 3

Total 100. 0

A glass made up of this composition is found to have a linear coeflicient of expansion of about 72 x 10" and this coeflicient of expansion remains, below about 550 C., substantially the same as the average coeflicient of the crystalline alumina abrasive grains. Above temperatures in the vicinity of 650 (3., the bond is sufliciently soft so that it may permanently deform and relieve any strains that may tend to be set up. Slow cooling in the annealing range, between say 650 and 550 C.. will remove strains from the glassy bond, although there is but little, if any, tendency for permament deformation in this range. The bond does not crystallize, if cooled with sufficient rapidity through the crystal forming range or until the bond begins to set, and therefore it does not crackle, nor does it craze at the line of junction with the alumina grains, hence the formation of internal strains and cracks is either obviated or to a large extent minimized. The bond matures to a relatively stable glass at tem- 'peratures belo'w l'175' C. detrimentally during such firing. Grinding I 1 and that the clearness of thering indicates i the absence of cracks'omdetrimental' flaws in .1 a

the wheel. I As an example which have a chemical analysis of 95.24

alumina, ferric oxid 0.743 silica 0.93, titania 3.09. One grade ofhar nessof wheel may operations after which it maybeffiredL A ceramist skilled in this'art will understand how to carry on the process of manufacture so as tov obtain the desired results, the different stages being in general first firing .to

vitrify'or maturethe bond, then cooling rapidly through the crystal forming stage, then slowly to efiect properannealing, and then cooling below the point of solidification of the bond to set the wheel ingredients permanently. A wheel of the above specified composition may be fired to Se er cone 4, ora

' temperature of approximate y 11251C. The

wheel is preferably cooled rapidly to about 800 C. and then slowly to about 600 C. where the bond is'solid. Below that temperature the bond and alumina abrasiye grains havle the same rate of shrinkage as the wheel coo s. 1

The wheel formed of the bond formula the bond to be relatively free from deleterious crystals and it matures at a temperature below the dissociation point of ferric oxid and below the temperature at which an appreciable amount of slag exudes from the abrasive grain into the bond. This bond shows but little tendency to swell during the firing operation and it is capable of bonding very coarse abrasive grains. as large as No. 8 grit and does wheel made up with this bond, we may ship oy. crystalline alumina grains in" grit sizes-from 24 to' 36' an integral body. above speclfied 1s very stable and hlghly 1nsoluble. A microscopic examination shows be apparent that variousotherroductsbesides grinding wheels may be-m s/ fi I i jand'sh'ape of the art cle,the type-of furnace employed and various other factors, and that the process will be so carried out as toobtainthe desired wheel structure. It will also ein accordance with this invention, and the claims are to be given a broad interpretation to cover ceramic, 'roducts having granular abrasive m r a terials, whatever purpose-the abrasive may be intendedto serve For-examp e, an article made in accordance with thisiinvention of fc'eramicbonded abrasive material,

such as :crystalline alumina, has various U f characteristics inherent in the hardness, durbe made by using one ounce of-this bondper pound of abrasive and the mixture-may be suitably wetted with water or 'a solution of a temporary binding material,-*andfltlie massshaped to ,the' desired wheel form by-s'uitable ability, and refractory qualities of the bonded mass and its selparate ingredients as well as other physica and chemical .pro erties, whereby the bonded product may employed as a safety tread, a porous. plate, a refracto brick or other .article adapted to with stan high temperatures, and for many other uses.' Ineach of these articles, the utilityof the bonded mass is greatly increased by providing a bond which has substantially the same coeflicient of expansion as that of the grair'n 'Having thus described the invention, what 7 we claim and desire to secureby Letters Patthe bond and unite the abrasive grains into 2. The method of makingan article of ceramic bonded granular material comprising the steps of mixing abrasive grains with a vitrifiable bond which matures at a temperature below that at which the constituents of the abrasive will deleteriousl affect the bond'and which has substantial y the same coefficient of expansion below the annealing range as that of the abrasive grains, and thereafter shaping an article therefrom and nded by vitrified "or glassy ma-' ains lot

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firing it to mature the bond and unite the abrasive grains into an integral body.

3. The method of making an abrasive article comprising'the steps of mixing abrasive grains with a vitrifiable bond which is so constituted that it will mature below 1200 C. without appreciable swelling taking place, and which, when vitrified, has substantially the same coeflicient of expansion below the annealing range as that of the abrasive grains, and thereafter firing an article shaped therefrom to mature the bond and unite the grains into a solid body.

4. The method of making an article of state has su tantially nseam ceramic bonded granular material comprising the steps of mixing crystalline alumina grains with a ceramic bond especially adapted for such ains which in the solid vitreous the same coeificient of expansion as that of crystalline alumina and thereafter firing the mixture under such conditions as will mature the bond into a glass which unites the grains into an integral body.

5. The method of makin an abrasive article comprising the steps 0 mixing crystalline alumina abrasive grains with a ceramic bond which is capable of maturing to a glass substantially free from crystal develo ment and having substantiall the same coe cient of expansion in the so id vitreous state as that of the abrasive grains and thereafter firing the article under such conditions as will mature the bond to a glassy condition, substantially free from deleterious crystals.

6. The method of making an article of ceramic bonded granular material comprising the steps of mixing ains of crystalline alumina with a ceramic 0nd which has substantially the same coeflicient of expansion as that of crystalline alumina and .which when matured to a solid glass will not be crazed or in a badly strained condition caused by a differential expansion between the bond and the crystalline alumina grains and which will mature below a temperature of 1200 C. and thereafter firing the mixture under such conditions as will mature the bond into a glass which unites the grains into an integral 7. The method of making an abrasive article comprising the steps of mixing the abrasive grains with a ceramic bond which has substantially the same coeflicient 0f expansion as that of the abrasive grains and is capable of maturing into an uncrazed glass at a temperature below 1200 0. without detrimental crystal development and thereafter firing the mixture to mature the bond and cooling it rapidly through the range of possible crystal development to minimize the formation of crystals.

8. The method of making an abrasive article comprising the steps of mixing crystalline alumina grains with a ceramic bond which is capable of maturing to a glass without swelling materiall and which has substantially the same coecient of expansion in the solid vitreous state as that of crystalline alumina and thereafter firing a mass shaped from such mixture under such conditions as will mature the bond and unite the grains into an integral structure.

9. An abrasive article comprising abrasive grains bonded by a glassy material which has softened sufiiciently below 1200 C. to adequately bond the grains together, and which when solid, has substantially the same coefiicient of expansion as that of the abrasive grains.

10. A grinding wheel comprising crystalline alumina grains bonded by a substance which has substantially the same coeflicient of expansion as that of crystalline alumina below the annealing range of temperatures and which is stable during normal usage.

11. A grinding wheel comprising cr stalline alumina grains united by a ceramic ond, in the form of a glass which has substantially the same coefiicient of expansion as that of crystalline alumina below the annealing range of temperatures and does not contain a detrimental amount of crystals, said bond being neither crazed nor crackled.

12. A bonded article comprising refractory abrasive granular material united by a vitrified ceramic bond in the form of a glass which has substantially the same coefiicient of expansion as that of the abrasive material when solidified and is so constituted and has been so fired that it is neither crazed nor crackled.

13. The method of making a ceramic article comprising the steps of mixing abrasive material with a vitrifiable bond which has substantially the same coeflicient of expansion as that of the abrasive material and which is capable of being matured without developing a material amount of deleterious crystals, and thereafter firing the mixture at such a temperature and cooling at such a rate as to mature the bond to a substantially homogeneous glass which is not crackled or crazed and in which there is no detrimental crystal development.

14. A. bonded article comprising granular abrasive material united by a bond which has been softened and solidified in position between the grains, said bond being so con.-

stituted that it has substantially the same coefficient of expansion as that of the abrasive material between the temperature of solidification of the bond and the normal temperature of use.

15. An abrasive article comprising abrasive ains united by an uncrazed and uncrac ed ceramic bond which have substantially the same coefiicients of expansion throughout the normal range of temperatures below annealing, said bond being a low melting glass, substantially free from deleterious crystals, which is capable of being matured below the temperature at which the constituents of the abrasive will deleteriously afiect the bond and at which swelling of the bond may take place.

16. An article of ceramic bonded granular material comprising abrasive grains of coarse size united by a glassy bond which have substantially the same coefficients of expansion, said bond being substantially uncrazed and not in a materially strained condition, as caused by differential expansion between the grains and the bond, and being uncrackledand substantially free from deleterious crystals.

17. An article of ceramic bonded granular material comgrising crystalline alumina grains united y a glassy bond which have substantially the same coeflicients of expansion, said bond being substantially uncrazed and not in amaterially strained condition,

' as caused by differential expansion between the grains and the bond, and therefore fit ting the grains below the annealin range of temperature, and being uncrackle' and substantially free from deleterious crystals a portion of the grains being of a size whlch will not pass through a screen having more than 36 meshes to the linear inch.

Signed at Worcester, Massachusetts, this 6th day of August 1927. LEWIS E. SAUNDERS. LOWELL. H. MILLIGAN. MILTON F. BEECHER. 

